In electrostatic printing and/or copying machines, a latent image is first produced on a latent image carrying means such as e.g. photoconductive surface of a photosensitive drum. A developer can be made of toner particles only (single component developer) or a mixture of toner and magnetic carrier particles b (two component developer). A developer is spread onto the latent image from a developer unit. Different imaging modes can be used such as Charged Area Development (CAD) or Discharged Area Development (DAD) as explained in “Electrophotography and Development Physics” 2nd edition 1988 by L. Schein (Springer Verlag) page 36. Using DAD, the toner is primarily attracted to those parts of the image which carry lower charge, typically as a result of imagewise discharge by an image exposure system, whereas the unexposed highly charged areas are not provided with toner. A toner image is so created on the latent image carrying means. The toner is manipulated in the developer by means of either its magnetic nature (single component magentic developer) or by means of the magnetic particles in the developer (two component developer) to place the toner into the correct state for printing or copying. Perfect control of the toner particles is required to prevent non-imagewise artifacts being generated in the image which are related to aspects of the developer and not the image. A medium on which the copy or the print is to be made, e.g. sheet of paper, plastic or cardboard, is then brought in juxtaposition with the toner image and receives a transfer of toner. The toner is then heated to bond the toner to the medium on which the finished copy or print is formed. Optionally, several toner images are made on several latent image carrying means, using toners of different colours, prior to transferring and binding the latent image to the finished copy or print by heating.
In one type of printer or copier, the toner is spread onto the latent image carrying means by using magnetic brushes. The magnetic brush is created on a developing roller which is part of the development unit that provides toner to the latent image carrying means. In this method of development, this occurs due to an electrostatic attraction between a charged toner and areas on the image carrying means, e.g. a photoreceptor. The development electrostatics can be adjusted so that development can take place in either the charged areas (CAD) or the discharged areas (DAD) of the image carrying means. Toner is added from a toner dispenser and it is mixed with magnetic particles called carrier particles. The toner is charged by triboelectricity and adheres to the carrier particles. A magnetic brush of developer particles is formed on a rotating sleeve surrounding magnets. The developer comprising the toner and magnetic carrier particles is attracted to the magnets and picked up by the sleeve. The magnetic carrier particles with attached toner form chains called a magnetic brush. The carrier is reused with new toner when toner is consumed in the image forming process.
In particular, in case of two component development systems using a developer comprising a mixture of (reusable) magnetic carrier particles and non-magnetic pigmented toner or toner particles for making a permanent image, these developing rollers comprise an internal magnet roller or discrete internal magnet configuration of permanent magnets or electromagnets and an outer sleeve, being the developing sleeve, which can rotate with or independently of the internal magnet configuration.
The permanent magnets typically may comprise rubber bond magnets or sintered rare earth magnets or combinations thereof.
Transport of toner is typically achieved by rotating the outer sleeve while the internal magnetic core remains static but alternative configurations exist where the internal magnet configuration is rotated in addition to a rotation of the sleeve.
The magnetic carrier particles, dressed with toner particles that are attached by electrostatic forces, form bead chains in interaction with the magnetic field as discussed for example in the Proceedings of the International Conference on Digital Printing Technologies, P. 742-747.
These bead chains create a magnetic brush on the sleeve. It is of importance to have a uniform magnetic brush with equally distributed bead chains over the sleeve surface.
The bead-chains of carrier particles dressed with toner, are magnetically attracted towards the outer sleeve surface of the developing roller by magnetic forces. The transport of the bead chains is believed to be the result of the magnetic interaction between the carrier particles and the magnet configuration, separated by the sleeve on one hand and the friction force between the sleeve surface and the carrier particles that contact the surface of the rotating developing sleeve on the other hand.
From U.S. Pat. No. 6,157,803 it is known that the surface condition or surface topology of the sleeve surface can influence the bead-chain build-up and development of the magnetic brush on the sleeve surface.
U.S. Pat. Nos. 4,018,187 and 5,153,376 teach to provide axially oriented grooves in the sleeve surface.
However there are some problems with the known rollers particularly when particular printing or copying conditions occur.
The provision of axially oriented grooves by presently known techniques causes the most demanding specifications of roundness and run-out for the sleeve to be met only with difficulty or even not to be met. This results in irregular distribution of toner particles in the final image.
In view of these issues there remains a need for cost-effective methods for making a developing sleeve having a good toner transfer characteristic, that can be used with durable materials such as non-magnetic steel and that also allow economic manufacture with the most demanding specifications of roundness and run-out for the sleeve as integrated in a developing roller for use in a printer or copier.